The present invention concerns circuits used to implement an integrating function and utilizable in adaptive delta modulation systems for companding purposes. Adaptive delta modulation is a familiar technique not requiring extensive review here, but reference may be had if necessary to e.g. the publication "Philips Technische Rundschau," 1970/71, No. 11/12, pp. 351-370.
In order to implement companding in a adaptive delta modulation system, it is conventional to vary the system's quantization step-size in dependence upon a control voltage derived from the delta-modulated output signal of the system. Such a control voltage is obtained using a logic circuit which receives the output pulses of the delta-modulation transmitter and in turn produces pulses whose repetition rate or frequency is a measure of the slope of the analog input signal received at the input of the delta modulation system. The pulses produced at the output of such logic circuit are applied to the input of an integrating circuit, and the latter produces at its output the quantization-step-size control voltage desired.
FIG. 1 schematically depicts a delta modulation transmitter in which companding is effected in accordance with such principle. The transmitter includes a difference or subtractor stage Di, one input of which receives the analog input signal w to be delta modulated. The other input of difference stage Di receives the approximation signal or reconstructed signal g of the system. The difference or error signal e =w -g produced at the output of difference stage Di is applied to a comparator S. The comparator S produces at its output a signal indicative of the sign (polarity) of the difference signal e. A sampling stage K, here a bistable circuit, samples this sign information with a sampling frequency fa. At the output of the sampling stage K there is produced the delta modulated signal d actually to be transmitted. The delta modulated signal d, in addition to being applied to whatever transmission channel L is employed, is applied to the input of a logic circuit LE, and also via a pulse converter IW to an input M1 of a multiplier M. The pulses produced at the output of logic circuit LE are applied to the input of an integrating circuit IN. The control voltage Us produced at the output of integrating circuit IN is applied to one input of an adding stage Ad, whose other input receives a voltage .DELTA.U which is added onto the control voltage Us. This voltage .DELTA.U corresponds to the smallest quantization step-size which appears in the case that Us=0. The output signal Us+.DELTA.U of the adding stage Ad is applied to an input M2 of multiplier M and serves to weight the constant-amplitude bipolar pulses produced at the output of pulse converter IW. The weighted-amplitude pulses produced at the output of multiplier M are applied to the input of an integrator I, at whose output the approximation signal g is produced, the latter being compared in difference stage Di against the analog input signal w.
The integrating circuit IN can, at simplest, be a simple RC low-pass filter. The time constant of such RC filter is dimensioned in correspondence to the characteristics of the type of analog input signal w to be employed. For example, when the input signal is always to be an analog speech signal, it is customary to assume that the amplitude of the speech signal's envelope will remain approximately constant within a time interval having the duration of one spoken syllable. During such time interval, accordingly, the quantization step size employed should vary at most only slightly. From such requirement it follows that the time constant for the RC low-pass filter should have a value of a few milliseconds. However, a capacitor capable of implementing such a time constant would require a large capacitance value, would be of unwieldy dimensions and could not be implemented in integrated-circuit technique.